Lactation
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Comparative Lactation - Marsupials

Marsupials

Lactation in Marsupials

This section discusses aspects of lactation in marsupials, including the evolution
of marsupials, neonate development, reproductive stratagy, lactation, and milk
composition.

Introduction

Marsupials evolved from ancestral therian mammals (which give birth to live young, but have no true placenta) over 80 million years ago; well before the demise of the dinosaurs. Eutherian mammals (which have a true placenta) also evolved from therian mammals, but evolved independently from marsupials. Although we are most familiar with marsupials like the American opossum and images of the Australian kangaroos and koalas, there are about 250 species of marsupials. However, in terms of lactation biology, most of what we know is derived from study of kangaroo species. Limited evidence from other marsupial species indicates that we can expect many similarities in strategies of lactation among most marsupials, so the kangaroo species makes an acceptable general model for much of the Order Marsupialia. Nevertheless, significant differences among marsupial species likely will be found as more species are studied.

Marsupial reproductive strategies are based primarily on the lactation phase, which is in contrast with eutherian mammals where intrauterine development of the young is the primary reproductive investment. The marsupial neonate is born at an extremely early stage of development (most weigh < 0.01 % of the mother's body weight at birth) relative to the neonate of eutherian mammals, even compared with bears and insectivores whose newborn are more developmentally advanced than any marsupial neonate. Newly hatched monotreme young are roughly as immature as the newborn marsupial.

Marsupials do seem to have the potential for an extended gestation, but this has not been exploited evolutionarily. This suggests that the emphasis on lactation, as opposed to intrauterine development, should be viewed as an alternate reproductive strategy and not as a more primitive form of mammalian reproduction. Despite the immature development of the newborn marsupial, they nevertheless can travel (unaided by the mother) from the birth canal to the mammary area or pouch and can attach to a teat and begin to suck. In addition, they are able to breath the atmosphere of the pouch.

Evolution of Marsupials

(one suggested scenario)

Marsupials probably initially evolved from therian ancestors in the mid-Cretaceous Period (about 80 million years ago) in North America. North American marsupials probably expanded to South America, Europe and the Pacific rim of Asia. Those in North America became extinct in the intervening time. Those reaching Europe became extinct during the Miocene Epoch (of the Tertiary Period). Species in Asia also did not survive to the present. South American marsupials, specifically Didelphis species, returned to North America in the Pliocene Epoch. This eventually differentiated to D. virginiana, the American opossum we are familiar with in the present. So, the American opossum is a relative newcomer to North America and not an immediate relative of the original North American marsupials. Other marsupials, also didelphid-like, crossed from South America to the Antarctic supercontinent which was linked to Australia. This occurred during the late Cretaceous or early Tertiary. Interestingly, other mammals did not successfully traverse across this Antarctic route, nor did monotremes from the Australian side traverse into South America. As Australia broke off the Antarctic and moved northward it was separated from other major land masses and sources of mammals by wide oceans. This allowed the marsupials of Australia to evolve independently. [For more information see Stonehouse and Gilmore]

Living marsupial are divided into about 16 Families representing about 250 species. These include:

Family

Type of Marsupials Represented

Approx. # of Species

Didelphidae

opossumes

70

Microbiotheriidae

Monito del monte

1

Caenolestidae

shrew-opossum

7

Dasyuridae

native cat, marsupial mice, Tasmanian devil

49

Myrmecobiidae

numbat

1

Thylacinidae

Tasmanian tiger

1

Notoryctidae

marsupial mole

1

Peramelidae

bandicoots

16

Thylacomyidae

rabbit-eared bandicoots

2

Phalangeridae

phalangers

11

Burramyidae

pigmy phalangers

7

Macropodidae

kangaroos

56

Phascolarctidae

koala

1

Vombatidae

wombats

3

Tarsipedidae

honey opossum

1

Neonate Development

One of the most studied marsupial species is the Tammar Wallaby (Macropus eugenii). This wallaby is born at 28 days of gestation, weighing 350-400 milligrams (mother weighs ~4-5 kilograms). The neonate remains permanently attached to one teat until Day 100 of lactation, during which time it grows slowly to about 100 grams. Brain development is disproportionately rapid during this period. After day 100 the rate of brain growth declines and functions necessary for the joey to leave the pouch begin to develop more rapidly, coinciding with an accelerated growth rate. At day 140 the eyes open and the underfur is visible. By day 160 the joey is able to stand unaided, kidney development is essentially complete and it is able to produce a concentrated urine. By day 180 thyroid function is fully developed and the joey is able to regulate its own body temperature. The joey may put its head out of the pouch to nibble grass at this time but it does not make its first trip from the pouch until about day 190. Once it leaves the pouch it repeatedly returns to the mother's pouch to suck for several more weeks. Peak milk intake does not occur until day 240, but herbage progressively forms a larger proportion of the diet. The joey leaves the pouch permanently at about 250 days and ceases to suck by day 300-350. [images provided by Dr. Kevin Nicholas, University of Melbourne, Australia]

[Image kindly provided by K Nicholas, University of Melbourne]

Mammary gland of a Tammar Wallaby. Note the newborn joey attached to one
of the nipples. Also note the long nipple that is still being succkled
by an older joey that is now staying outside of the pouch.

Newborn joey. Note the immature stage of development and that it is "attached" to the nipple.

A joey at 70 days after birth. At this stage the joey would still be
in the pouch and attached to the nipple. Note the well developed claws.

A 150 day old joey. The joey would still be residing in the mother's
pouch.

A 200 day old joey. At this stage it would be staying outside of the
pouch, but continuing to nurse the mother by sticking its head into the
pouch. See the long nipple in teh image above of the mammary gland.

An adult Tammar Wallaby. What is she thinking about?

Reproductive Strategy

Shortly after the first joey is born, the mother mates again. This new embryo stays at an early dormant stage of development until the first joey begins to leave the pouch. The decline in suckling stimulus associated with the first joey leaving the pouch allows the second joey embryo to develop, be born and enter the pouch. The mother may then mate again and can keep the third joey in the arrested embryonic stage until the first joey is completely weaned and the second joey has started leaving the pouch. This way a female kangaroo can support three offspring, each at a different stage of development. This allows kangaroos to rapidly repopulate after a drought (a very common occurrence in central Australia) or other disaster. A mother kangaroo can produce 25 offspring in her lifetime, but usually there is high attrition due to extreme droughts.

Lactation

Mammary function or lactation is divided into three phases for Macropus eugenii. Phase 1 is actually comparable to mammary development during gestation in eutherian mammals. Phase 2 is the early period of milk secretion when the joey is still in the pouch. Phase 3 coincides with the joey beginning to leave the pouch. Mammary development occurs during pregnancy, however the mammary gland is relatively underdeveloped at the time of birth compared with most eutherian mammals. By day 10 of pregnancy the lobuloalveolar pattern of development is evident in all 4 glands.

Lactogenesis is the transition from phase 1 to phase 2 and occurs in all 4 glands. Actual milk secretion starts about 24 hr postpartum. Because the joey attaches to only one teat, only that teat continues to develop while the other 3 glands regress. When the mother has an older joey out of the pouch the newborn will choose one of the remaining 3 teats. The other two glands will regress and the gland of the teat to which the neonate attaches will develop independently of the gland suckled by the older joey. Both the mammary gland and the teat continue to develop and grow as the newborn joey increases in body size. The gland continues to develop more secretory alveoli, while the teat also gets larger (2 to 3 mm length at parturition vs. about 23 mm by day 180 of lactation).

A considerable change occurs in mammary size and function in the transition from phase 2 to 3, concurrent with the joey beginning to leave the pouch. There is a shift in milk composition, a rapid increase in rate of growth of the gland and an increase in total milk secretion.

Control of lactogenesis at the peripartum period apparently is not under the inhibitory control of progesterone as in eutherian mammals. Nevertheless, progesterone is still necessary for mammogenesis during pregnancy. It is unknown what prevents lactogenesis from occurring in late pregnancy in the species. Prolactin seems to be the primary hormonal regulator of lactogenesis and lactation in marsupials. Although suckling by the neonate during phase 2 lactation does not stimulate prolactin secretion (as it does in eutherian mammals), it does apparently induce the synthesis of prolactin receptors in the mammary gland and thereby make that gland more sensitive (responsive) to the low blood concentrations of prolactin. Of course, this means that only the suckled gland becomes more responsive to prolactin. This is an example of local control of one gland independent of the others.

Phase 3 mammary tissue histologically looks like fully lactating tissue of eutherian mammals, while phase 2 tissue looks more like the prepartum mammary tissue. Binding of prolactin is significantly further enhanced during the 3rd phase compared with the 2nd phase. In addition, concurrent with the transition from the 2nd to 3rd phases of lactation, there is a transient increase in prolactin concentration in the blood.

Oxytocin stimulates milk ejection in marsupials as in eutherian mammals. However, there is a progressive decline in sensitivity of the lactating gland to oxytocin as lactation progresses. This means that for the early lactation gland with a small joey attached (and therefore light but continuous stimulation of the milk ejection reflex and oxytocin secretion by the pituitary) the myoepithelial cells can respond to very low levels of blood oxytocin (ie. the gland is very responsive). Any gland at a later stage of lactation would not respond the these low oxytocin levels. In contrast, in late lactation when the joey returns to the teat to suckle intermittently, but vigorously, a greater release of oxytocin may occur causing milk ejection in that gland.

Milk Composition

Although changes in milk composition occur during lactation in eutherian mammals, those changes are minor compared with the changes observed in marsupial milk. During early lactation (phase 2) carbohydrate is high (50% of the total solids) and lipid is low (~15% of total solids) in milk of Tammar Wallabys. At the transition from phase 2 to 3, carbohydrate is reduced to very low concentrations while lipid content may exceed 60%. At lactogenesis, lactose is synthesized, but by day 7 after birth a second galactosyltransferase activity appears in the mammary cells and adds additional galactoses to lactose resulting in tri- to penta-saccharides. By day 182 most milk sugars are oligosaccharides. Then in the 3rd phase of lactation the low levels of milk sugars are predominantly monosaccharides.

Total milk protein is relatively constant throughout lactation, at about 25% of solids, although it may transiently increase around the time of transition from phase 2 to 3. During phase 2 whey proteins account for 50 to 85% of total milk proteins, but the proportion of casein may increase in phase 3 milk. Secretion of at least one additional whey protein is induced during the transition from phase 2 to 3. The function of this whey protein is unknown.

These changes in milk composition occur independent of what is happening in an adjacent gland. So, when the mother is nursing joeys of two different ages she is simultaneously producing two types of milk of very different compositions, one for the joey in the pouch and one for the older joey that is outside of the pouch.